I just went through a battle finding the reason for my water pump losing prime.
Here was the symptom.
You turn on a faucet and it takes about 30 to 45 seconds before it primes. You then shut it off and it comes to pressure and shuts off.
Now, you wait maybe 1 minute and turn it on and it has lost prime and takes about 30 seconds to reprime. I started to check from the intake backwards and resealed each connection one at a time and then retry.
Let me pause here:
We have a 2008 Jayco Melbourne 29D. It has a water distribution center for different choices, such as CITY WATER, CITY FILL, COUNTRY FILL, NORMAL OPERATION ETC.
I back tracked to the distribution manifold and found where the pump intake line was connected. It is a nylon nipple on one end to thread on the other and threads into a nylon female connection. I took this line off and unthreaded the nipple, put new teflon pipe thread tape on and reinstalled it. WOW!!!! Now it held prime. Even all night.
The hardest part was getting to the water pump area. Had to cut the control board in half and unscrew the panel that held the valving. Hope this will help someone somewhere. Remember the symptoms!!!
There could be other reasons for losing prime but with these symptoms, you know its a leak in the intake line sucking air and letting the water syphon back in the line.